This invention relates to a method for treating polyvinyl chloride resins to reduce the vinyl chloride monomer content thereof.
In conventional processes for producing polyvinyl chloride resins, the unreacted vinyl chloride monomer (VCM) is removed from the resin in a number of different steps in the finishing process. In the suspension process, a large portion of the unreacted vinyl chloride monomer may be removed by evacuating the polymerization autoclave to adiabatically flash the unreacted monomer. Alternatively, the suspension resin-monomer-water slurry produced in an autoclave may be transferred to a degassing vessel and vacuum applied to adiabatically flash the monomer which is then condensed or otherwise recovered. Further reductions of the monomer content of the resin-water slurry can occur in vented holding or staging tanks wherein the product from the autoclave or other source is held for some period of time prior to further processing. The vinyl chloride monomer content is significantly lowered in the final dried PVC resin product during the drying step. The PVC slurry is separated into aqueous and wet solid components by conventional means such as centrifuging and/or filtering. The wet solids are then dried by conventional means such as a rotary dryer or a dryer which blows heated air through the resin particles as they are conveyed through a closed system or while they are contained in a drying vessel. Conventional suspension polyvinyl chloride resins dried according to the foregoing conventional procedures have been found to contain, for example, from about 25 parts per million to about 4,000 parts per million vinyl chloride monomer in the final dried resin product, depending upon the particle characteristics of the particular resin produced.
Recently the vinyl chloride monomer content of polyvinyl chloride resins has become a matter of great concern to the public, the government, and the industrial producers of these resins. Governmental agencies are proposing maximum levels of vinyl chloride monomer in air surrounding polyvinyl chloride production facilities. Regulations are also proposed for limiting the emission of vinyl chloride monomer to the atmosphere. Additionally, standards for maximum levels of vinyl chloride monomer extracted from PVC containers such as bottles or jars used to package food or cosmetic products are anticipated.
In view of the foregoing, it is seen that there is a pressing need for a process for both reducing the vinyl chloride monomer emissions from suspension polyvinyl chloride plants and also the vinyl chloride monomer content of the PVC suspension resins which are produced by these plants.
In copending patent application Ser. No. 498,547, filed Aug. 19, 1974, and entitled "Treating Polyvinyl Chloride Resins" (assigned to the same assignee as this application), now abandoned, there is disclosed a process for reducing the vinyl chloride monomer content of suspension PVC resins. The process of the aforementioned application involves heating an agitated PVC resin-water slurry in a closed vessel to a temperature of at least about 50.degree. C while applying vacuum to the vessel for a period of time sufficient to reduce the vinyl chloride monomer content of the resin and then cooling the slurry. The vacuum stripped slurry is then dewatered and dried. This process suffers a disadvantage in that it requires a considerable amount of time to achieve a reasonable reduction in the vinyl chloride monomer content of the resin. Further, the process does not achieve as great a reduction as is desired in the vinyl chloride monomer content of the types of suspension PVC resins that are utilized in producing PVC compounds used for manufacture of containers, such as blow molded bottles, employed for packaging food, drug or cosmetic products. The process of the present invention overcomes the aforementioned disadvantages in that it achieves a more efficient removal of vinyl chloride monomer from the suspension PVC resin in a much shorter period of time.
Heretofore, it had been generally assumed that PVC suspension resins could not be subjected to temperatures approaching their glass transition temperature without resulting in a degradation in the quality of the PVC resin. Surprisingly, the process of the present invention wherein the PVC resin may be heated to temperatures in excess of its glass transition temperature does not produce any significant deterioration in the quality of the PVC resins. In fact, some types of PVC resins appear to have improved qualities after being subjected to the steam stripping process of the present invention.